SD card library is not being found.

I've tried to use the Arduino built-in SD card library. In my code, I have:

```
find_arduino_library(sd_lib SD ${board_id})
```
Which raises an error:
```
  Couldn't find library named Sd
```

I think it's somehow CamelCasing it, which is causing it to not find the actual path, `libraries/SD/`. The `SPI` library works fine, though, so it's a bit confusing.